Heterojunction bipolar transistor integrated circuits are used in a wide variety of applications including satellite telecommunication systems, high-speed analog/digital converters, wireless communications circuits, radar systems, and Microwave Monolithic Integrated Circuits (MMICs). Silicon-Germanium heterojunction bipolar transistors exhibit significantly enhanced performance relative to silicon homojunction transistors. Silicon-Germanium heterojunction bipolar transistors offer a lower barrier for electron injection into the base as compared to homojunction devices.
A problem common to npn Silicon-Germanium devices is the out-diffusion of boron from the Silicon-Germanium component. This out-diffusion can result from a number of factors including transient enhanced diffusion resulting from an annealing step. The boron out-diffusion into the silicon region is undesirable, in part, because it results in the formation of a parasitic conduction band barrier thereby reducing the transistor's gain and speed.
In an attempt to overcome the problems associated with boron out-diffusion, undoped SiGe spacer layers have been deposited on either side of the boron-doped SiGe base layer. This previous technology is shown schematically in FIG. 1a. The boron-rich region 100 is flanked by two SiGe spacer layers 102, the spacer layers 102 are included in anticipation of boron diffusion. Such an approach results in a base layer thickness 104 that may be twice as thick as the base layer provided for in the present invention.
An alternate approach, according to the literature, for limiting boron diffusion has been to dope the boron doped SiGe base layer with carbon 110. This approach, while generally effective in preventing boron out-diffusion, has serious limitations. First the carbon concentration necessary to mitigate the effects of boron diffusion may necessarily exceed 1.times.10.sup.19 cm.sup.-3 with as much as 50% of the carbon located interstitially. This level of interstitial carbon can reduce the mobility, and hence the speed, of SiGe HBTs by as much as a factor of 10. This reduction results, in part, from the fact that SiGe has a higher hole mobility than SiGe doped with carbon, which is due to the lack of carbon-related complexes associated with interstitial or otherwise non-substitutional carbon. Additionally, traps associated with non-substitutional carbon can affect the position of the Fermi level in the base, which also adversely affect device performance. None of the existing methods allows for the concurrent enhancement of base mobilities and for substantial elimination of boron out-diffusion.